Read-only-memories (ROMs) are well known in the semiconductor art, and are programmed during processing by generating a special interconnect (metal, contact, doped polycrystalline silicon, etc.) mask which corresponds to the data to be stored on the chips. The data or program is essentially "hard-wired" into the integrated circuit structure and cannot be changed after fabrication is completed.
ROM technology is characterized by the fact ROM chips can be produced in a short turn-around time. That is, the time that a customer orders a ROM chip with a particular set of data or program to the time the chip is delivered is relatively short as compared with other integrated circuit chips since the programming can take place late in the process. Further, ROM chips can be made with high density which provides low cost per bit of storage. Additionally, ROMs provide memories with short access times.
It is known that one goal in ROM technology is to increase the density of the memory by storing more than a simple binary quantity in each bit. Instead of each bit having simply an "on" and an "off" state, each cell would have several "on" states and one "off" state. Each "on" state corresponds to a different level of current during a read operation. Such cells are called multi-state ROM cells.
It is understood that multi-state ROM cells have been proposed using cell elements of different geometries. However, such cells have disadvantages in that in their fabrication, the critical dimensions, or smallest resolvable dimensions of the process, must be tightly controlled to provide such geometric devices to be precisely made. Further, with this approach, programming must be done fairly early in the IC fabrication and thus the turn-around time would be lengthened, which is an additional disadvantage. Some multi-state ROM cells also consume considerable lateral substrate space, which is also a disadvantage in high density integrated circuits.